It is frequently useful, especially for satellite communication, to provide a beam of generally elliptical cross-section having different widths in two mutually orthogonal planes. The reduction of the beam width in one of these planes, with maintenance of a desired spread in the other plane, saves energy and minimizes interference with radiation from other sources.
There are several ways of controlling the shape of a beam representing the major lobe of a radiation pattern. One method is based on the structure of the primary source or feed and involves an intricate shaping of the feed aperture, a multimode excitation of the feed or the use of several sources excited in a predetermined phase and amplitude relationship. A second method uses a reflector shaped according to optical principles which, in their application to microwave transmission, generally require a subdivision of the reflecting surface into discrete points or lines conforming to certain geometrical relationships. The construction of such a reflector is difficult since, aside from its electromagnetic performance, it must likewise satisfy certain mechanical requirements in order to have the necessary structural stability. There is also the possibility of providing the reflector with a shield leaving an aperture of the desired shape, yet this expedient alone is usually unsatisfactory and must generally be supplemented by other measures of the type referred to above.